Energetics of Twisting in the Lowest Stilbene Triplet State

Article abstract of DOI:10.1021/ja00364a027

The temperature dependence of the azulene effect on the benzophenone-sensitized photoisomerization of the stilbenes is reported in toluene, benzene, tert-butyl alcohol, and acetonitrile.The data are consistent with formation of identical stilbene triplets from the two stilbene isomers.Predictions concerning the shape of the potential energy curve for twisting about the central bond depend on the detailed mechanism assumed for the quenching interaction.A lower enthalpy for the twisted triplets is predicted, ΔH_tp = -2.1 and -1.6 kcal /mol in toluene and tert-butyl alcohol, respectively, if it is assumed that the equilibrium population of transoid triplet geometries, ³t^*, is negligible and that quenching follows encounters of twisted stilbene triplets, ³p^*, with azulene when they achieve ³t^* geometries in the encounter cage.On the other hand, if quenching is assumed to occur only by direct interaction between ³t^* and azulene, equilibrium constants for the ³t^* ->/<- ³p^* process are obtained which predict a lower enthalpy content for transoid triplets: ΔH_tp = 0.5 and 2.9 kcal/mol for toluene and tert-butyl alcohol, respectively.In the latter case entropy is predicted to favor twisted geometries so that for both mechanisms the free energy favors ³p^* at the temperatures normally employed for photochemical studies.Reasons for favoring the ³p^* quenching mechanism are presented.